Clock generators are used in a variety of integrated circuits. For example, receiver chips use clock generators for local oscillator signals to mix a radio frequency (RF) signal to an intermediate frequency (IF) or to baseband. Transmitter chips use clock generators to generate carrier waves for RF signal transmission. Since these integrated circuits can tolerate very little frequency error, they typically use external crystals to generate precise mixing or carrier clock signals.
However crystal-based oscillators have drawbacks. For example, crystals are electromechanical devices that are trimmed to resonate at a particular frequency, and they are not compatible with modern CMOS integrated circuit manufacturing technologies. Thus the use of discrete crystals for crystal oscillators increases printed circuit board area and cost due to the added cost of the crystals. In addition, the connections between the integrated circuit and the external crystal pick up electromagnetic signal energy, causing phase jitter. Moreover there are some circumstances in which the frequency of a crystal oscillator will deviate significantly from its rated frequency. For example, as a printed circuit board heats and cools, the solder compound may not expand and contract at the same rate as the printed circuit board substrate. The result is that heating and cooling exerts mechanical stresses on the integrated circuit pins used to connect to the crystal. This condition, known as solder shift, causes the capacitance on the terminals of the crystal to change, which causes the oscillation frequency to shift due to the change in capacitance, but in generally unpredictable ways.
The use of the same reference symbols in different drawings indicates similar or identical items. Unless otherwise noted, the word “coupled” and its associated verb forms include both direct connection and indirect electrical connection by means known in the art, and unless otherwise noted any description of direct connection implies alternate embodiments using suitable forms of indirect electrical connection as well.